The signals which are modulated in particular to the carrier frequency, and which are transmitted by one or several transmitter sources, usually include messages which have to be demodulated in the correlation and demodulation circuit. The code which modulates said signals to define the transmitter source is usually a pseudo-random code. This determined repetition length code is unique for each transmitter source so as to be able to recognise which transmitter source the signals received by the receiver are coming from. They may be for example signals in the telecommunication field or positioning signals using satellites, such as GPS type signals.
In the case of GPS receivers, the radio-frequency signals are transmitted by several satellites placed in orbit which are distinguished from each other by a specific pseudo-random code, called the Gold code, modulated on the carrier frequency signals. The Gold code is a digital signal which is formed of 1023 chips and which is repetitioned every millisecond. The chips have values of 1 or 0 like a bit. All the Gold codes have the characteristic of being orthogonal which consequently means that by correlating them with each other the correlation result gives a value close to 0. This characteristic allows several radio-frequency signals simultaneously coming from several satellites to be processed independently for example in several correlation channels of the same receiver.
The GPS signals supply position and time data to a receiver for calculating operations of the X, Y and Z position, speed and universal time. However, in order to determine position and time, the receiver has to obtain data from at least four visible satellites.
In various fields of use of specific code modulated signal receivers, such as radio-frequency signals, one has to assure that the parts of the receiver which allow in particular the messages to be extracted from the received signals, work properly. Operating tests may be performed on said receiver parts either before or after the assembly thereof. Of course, said receiver parts have had to undergo several preliminary steps of elementary tests before their operation can be checked generally.
During use of a GPS receiver, an operating test of said receiver has to be performed in close to real conditions in order to assure that each channel of a correlation stage is working properly. If this operating test is successful, this allows the validity of the position, speed and time calculations for example to be guaranteed to the user of the receiver.
By way of example, in particular in the field of telecommunications, U.S. Pat. No. 4,100,531 discloses means for measuring the bit error rate of digital equipment, such as a data transmission device. This equipment is tested using pseudo-random code (PRN code) test signals supplied by a transmitter and received by a receiver to be tested. The receiver tested generates a PRN code replica to be correlated in a correlation stage with the signals modulated by the PRN code of determined length.
One drawback of this equipment is that the encoded test signals are sent by an external transmitter to the receiver in order to be tested, which does not allow the test time to be significantly reduced. Moreover, another drawback is that the transmitted signals have to be the image of the real communication signals to constitute a test representative of the receiver's operation. This inevitably involves having additional noise on the test signals, which is often difficult to produce.
In the field of GPS type receivers, U.S. Pat. No. 5,093,800 discloses a test apparatus capable of generating GPS type radio-frequency signals. These GPS signals generated by the apparatus are intended to be received by a GPS receiver to be tested. In order to do this, the apparatus also includes data relating to the satellites so as to be able to generate and transmit signals corresponding to the encoded signals transmitted by several satellites to the receiver.
One drawback noted with this test apparatus lies in the fact that in order to be able to check that the correlation stage of the receiver is working properly, the test signals are radio-frequency signals equivalent to the radio-frequency signals transmitted by satellites. This obviously involves generating additional noise with the radio-frequency signals generated in the apparatus in order to check that the correlation stage is working properly. Moreover, another drawback is that the receiver operating test time is relatively long since it is dependent on the repetition length of the pseudo-random code, i.e. the Gold code. If the receiver is mounted in a device powered by a battery or an accumulator, the long test time may also run down said battery or said accumulator unnecessarily.